The primary goal of the proposed study is to assess the genetic etiology of reading disability (RD) and comorbid Attention-Deficit/Hyperactivity Disorder (ADHD), the two most prevalent disorders of childhood. To accomplish this goal, an extensive psychometric test battery, including the WISC-IV, tests of reading performance and related cognitive processes, and parent and teacher ratings of ADHD, will be administered to a sample of 1,650 children from 600 families in which at least one of two or more siblings has a school history of reading difficulties. DNA will be isolated from blood or buccal cell samples obtained from all siblings and both biological parents, and each sample will be genotyped using a procedure that provides genotypes for over 10,000 single-nucleotide polymorphisms in a single assay. These extensive phenotypic and genotypic data will be used to accomplish three goals. First, a genome scan will be conducted in the largest sample of sibling pairs ascertained for RD that has been collected to date. This analysis will attempt to replicate the localization of quantitative trait loci (QTLs) for RD that have been reported in previous studies, and is also expected to reveal new QTLs that have not been detected in previous genome scans. Second, bivariate linkage analyses will be employed to conduct the first genome-wide scan for QTLs with pleiotropic effects on RD and ADHD. Finally, exploratory analyses will be conducted to capitalize on other aspects of the rich phenotypic dataset, including a univariate genome-wide scan for DSM-IV ADHD in this enriched sample of siblings ascertained for reading difficulties.